Microsoft is breathing more life into older versions of its products by offering businesses extended coverage for fixing security problems and bugs. Large companies running old favorites Windows NT 4.0, Exchange Server 5.5 and Windows XP Service Pack 1 can expect at least three more years of support from Microsoft after it decided to update the Custom Support Agreement program. There is even the suggestion support will go beyond three years.

while NT4 best fit SMB needs, windows 2000 was a good upgrade to the NT kernel with good PnP support and nice APIs.
NT(3.51 & 4) was really a neat a simple OS, with multiple processor support ( Alpha, Power PC ) and a good break away from DOS while keep a good compatibility layer with windows.